grep

This is similar in spirit to, but not the same as, grep(1) and its
relatives. In particular, it is not limited to using regular expressions.

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_
to each element) and returns the list value consisting of those
elements for which the expression evaluated to true. In scalar
context, returns the number of times the expression was true.

Note that $_
is an alias to the list value, so it can be used to
modify the elements of the LIST. While this is useful and supported,
it can cause bizarre results if the elements of LIST are not variables.
Similarly, grep returns aliases into the original list, much as a for
loop's index variable aliases the list elements. That is, modifying an
element of a list returned by grep (for example, in a foreach
, map
or another grep) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.

If $_
is lexical in the scope where the grep appears (because it has
been declared with my$_
) then, in addition to being locally aliased to
the list elements, $_
keeps being lexical inside the block; i.e., it
can't be seen from the outside, avoiding any potential side-effects.